I have heard this comment about grout colours before and some just say to pick a shade or two darker to compensate. What about contacting those brands and asking them?

Who knows, maybe there’s something inherent in some tap waters that commonly can skew colour consistencies. Waters are different everywhere for a variety of reasons. If it’s well water, there are minerals like manganese and sulphur that I can see as possible tinting culprits. How about trying distilled water next time?